This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 11-186548, filed Jun. 30, 1999, the entire contents of which are incorporated herein by reference.
The present invention relates to a field emission device comprising means for limiting the current at the time of occurrence of a short circuit between the emitter and the gate.
Recently, the field emission device of the field emission type that is as small as the semiconductor device has been developed by use of the developed Si-semiconductor micromachining technique, and the application of the field emission device to a flat panel display and the like has been promoted. The report of C. A. Spindt et al. (Journal of Applied Physics, vol. 47, 5249, 1976) is known as a typical example of it.
As for the field emission device described in this report, as shown in FIGS. 1A to 1D, an SiO2 layer 2 is formed as an insulation layer on an Si-monocrystalline substrate 1 by thermal oxidation. After an Mo layer 3 that is to be a gate electrode is formed by vacuum evaporation, a hole 4 is formed by etching (FIG. 1A). Then, Al is deposited by vacuum evaporation in the oblique direction while rotating the Si-monocrystalline substrate 1, and an Al layer 5 is thereby formed (FIG. 1B).
Next, Mo that is to be an emitter is deposited by vacuum evaporation on the Si-monocrystalline substrate 1 in the vertical direction, and Mo is stacked in a conical shape inside the hole 4 by taking advantage of the fact that the hole 4 becomes closed as a Mo layer 6 is stacked (FIG. 1C). Finally, a conical emitter 7 is formed by removing the Al layer 5 and the Mo layer 6 (FIG. 1D).
A resultant structure is held face to face with an anode (not shown) in a vacuum vessel so as to be the field emission device.
This field emission device applies the positive voltage to the gate to generate a large electric field at the tip of the emitter, and the electrons extracted from the interior of the emitter into the vacuum is collected by the anode (not shown). Actually, the field emission device has an array structure comprising many emitters to obtain a large current. However, as a large number of emitters are connected in parallel, there is a serious problem that the overall array cannot be operated if one emitter causes a short circuit with the gate. Thus the redundancy in a short circuit between the emitter and the gate needs to be improved.
To solve this problem, the device having the structure proposed in Extended Abstracts (The 54th Autumn Meeting, 1993): The Japan Society of Applied Physics 27P-Y-9. The structure of the device is shown in FIGS. 2A to 2C. In the device shown in FIG. 2A, a lower part of an emitter 10 is formed by a high resistance layer 11. In FIG. 2B, a gate potential is supplied to the high resistance gate 15. In either case, when a short circuit occurs between the gate and the emitter, most of the voltage is applied to the high resistance portion and the overall device is prevented from becoming inoperable to maintain the redundancy.
In the device of FIG. 2C, the array is divided into blocks 17 and the gate potential is supplied to the gate of each block through a fuse 18. When a short circuit occurs, the fuse is melted to electrically separate the blocks and the redundancy is thereby maintained.
A device having the structure as described in the Technical Digest of IVMC, 91, p. 200, 1991 is also proposed. The structure is illustrated in FIG. 3.
After an n-type source region 21 and an n-type drain region 22 are formed on a p-type Si-substrate 20, a thermally oxidized SiO2 layer 23 is formed. Next, an emitter is formed in the above-explained manner in a drain region of a MOSFET produced by forming a source electrode 24 and a gate electrode 25. In this device a current flowing in the emitter can be controlled by the MOSFET inserted at the emitter side. For this reason, a current flowing at the occurrence of a short circuit between the emitter and the gate is limited by the FET and the redundancy can be therefore maintained.
However, there is a serious problem in the above-described conventional method as explained below.
First, according to the method of limiting a current at the occurrence of a short circuit by a large resistance, such a resistance is needed as to be able to suppress the current in a short circuit. As a result, the operating speed of the device becomes remarkably lower. Further, when the resistor is inserted at the emitter side, there is another problem that the resistor causes the loss to be increased because of a large emitter current when the device is normally operated.
There is no problem about the resistance in the case of the fuse. However, it is difficult to produce a fuse blown at a low voltage with a small current, integrally with the substrate. For example, to prevent the heat from being diffused from the fuse, the fuse needs to be positioned apart from the substrate.
In addition, there is another problem that the blocks are separated at a low speed as the opening of the fuse requires a long time.
As for the structure of inserting the MOSFET at the emitter side, when the device is normally operated, the loss caused by the FET becomes large because of a large emitter current though the loss is not so serious as that in the case of the resistor. Further, as the ability to treat a large emitter current is required of the FET, a large area is needed for the FET and thereby the packing density of the device cannot easily be increased.
The structure of inserting the MOSFET at the emitter side has an advantage of simple production as the emitter can be formed on the drain region of the MOSFET as described above. When this structure is applied to a display device the display requires a comparatively small current and, therefore, the problem of the loss caused by the FET is not so serious. Furthermore, there is an advantage that the brightness does not vary as it is determined in accordance with the characteristics of the FET.
On the other hand, in a case where the field emission device is applied to the power switching device, the loss in the MOSFET is a serious problem when the MOSFET is inserted on the emitter side. For this reason, means for maintaining the redundancy relating to a short circuit between the emitter and the gate is required in the power switching device.
The object of the present invention is to provide a field emission device which is capable of certainly separating a short-circuited portion at a high speed without causing the lowering of the operating speed of the field emission device or the increase in the power loss, and which is thereby suitable for a power switching device.
To achieve the object, a field emission device according to a first aspect of the present invention comprises:
a casing whose interior is kept to be a vacuum;
an anode plate provided inside the casing;
an emitter plate for emitting electrons held to face the anode plate in the casing, the emitter plate being composed of a conductive plate having a plurality of field emission portions facing the anode plate, the electrons being emitted to the anode plate so that a current flows between the anode plate and the emitter plate;
a gate plate for controlling the current between the anode plate and the emitter plate, the gate plate having a plurality of openings corresponding to the plurality of field emission portions of the emitter plate, the emitter plate and the gate plate being held to be insulated from one another;
a gate voltage supply terminal for supplying a gate voltage to the gate plate; and
a gate current limiting element inserted between the gate plate and the gate voltage supply terminal.
A field emission device according to a second aspect of the invention comprises:
a casing whose interior is kept to be a vacuum;
an anode plate provided inside the casing;
an emitter plate for emitting electrons held to face the anode plate in the casing, the emitter plate being composed of a conductive plate having a plurality of field emission portions facing the anode plate, the electrons being emitted to the anode plate so that a current flows between the anode plate and the emitter plate;
a gate plate for controlling the current between the anode plate and the emitter plate, the gate plate being made of a first conductivity type semiconductor having a plurality of openings corresponding to the plurality of field emission portions of the emitter plate, the emitter plate and the gate plate being held to be insulated from one another;
a second conductivity type semiconductor layer arranged around the plurality of opening portions of the gate plate, separately from the emitter;
a gate voltage supply terminal for supplying a gate voltage to the gate plate; and
a gate current limiting element inserted between the gate plate and the gate voltage supply terminal.
A reverse bias is applied between the first conductivity type gate plate and the second conductivity type semiconductor layer.
The first conductivity type is preferably an n-type and the second conductivity type is a p-type.
The field emission device according to the first and second aspects is preferably constituted as follows.
The gate current limiting element is composed of an active element for limiting a current when a short circuit occurs between the emitter plate and the gate plate.
The gate plate is composed of a semiconductor material, and the active element is a junction FET integrally formed with the gate plate.
The FET is integrally formed with the gate plate in the same layer.
The FET has a gate, a source and a drain, the drain is connected to the gate voltage terminal, and the gate and the source are both connected to the gate plate.
The active element is a MOSFET integrally formed with the gate plate.
The MOSFET is integrally formed with the gate plate in the same layer.
The MOSFET has a gate, a source and a drain, the drain is connected to the gate voltage terminal, and the gate and the source are both connected to the gate plate.
The gate plate is divided into a plurality of blocks and each of the plurality of blocks has the current limiting element.
The device is a power switching device.
The field emission device of the present invention comprises a current limiting element for electrically separating the blocks including the device where a short circuit occurs between the gate and the emitter, on the gate side to which only a small current flows in the normal operation. For this reason, the loss in the normal operation is so small that it can be neglected, and the current limiting element does not have to have an ability to treat a large current. Lowering of the operating speed of the field emission device in the normal operation can be made smaller as compared with a case where a resistor is used. Further, the current limiting element can easily be produced integrally with the field emission element by the conventional semiconductor manufacturing technique.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.